1. Field of the Invention
The present invention relates to a binder composition for a magnetic recording medium. More particularly, the present invention relates to a binder composition for a magnetic recording medium containing a binder resin that is suited to the manufacturing of magnetic recording media having both good electromagnetic characteristics and running durability.
The present invention further relates to a magnetic recording medium containing the above binder resin.
2. Discussion of the Background
In particulate magnetic recording media, the binder plays important roles with respect to electromagnetic characteristics, running durability, and the like.
Vinyl chloride resin, polyurethane resin, polyester resin, acrylic resin, and various other resins are employed as binders in magnetic recording media. Of these, vinyl polymers such as vinyl chloride resin and acrylic resin are widely employed due to the high degree of unit freedom, ease of conducting the synthesis reaction, and the like they afford (see Document 1: Japanese Unexamined Patent Publication (KOKAI) Heisei No. 8-67855; Document 2: Japanese Unexamined Patent Publication (KOKAI) No. 2004-295926; Document 3: Japanese Unexamined Patent Publication (KOKAI) Heisei No. 6-111277; and Document 4: Japanese Unexamined Patent Publication (KOKAI) No. 2005-310332, which are expressly incorporated herein by reference in their entirety).
However, investigation by the present inventors has revealed that conventional vinyl polymers, including the polymers described in Documents 1 to 4, do not have adequate characteristics as binders in magnetic recording media for high-density recording for the following reasons:
(1) The use of microparticulate magnetic powder and dispersion of the microparticulate magnetic powder to a high degree to increase the smoothness of the magnetic layer surface are effective for achieving high-density recording. Further, increasing the dispersibility of the nonmagnetic powder contained in the nonmagnetic layer positioned beneath the magnetic layer is also an effective means of increasing the smoothness of the magnetic layer surface. Accordingly, adsorption functional groups (polar groups) such as SO3Na groups are widely incorporated into the binders employed in magnetic recording media. The use of finer microparticles in both the magnetic material and nonmagnetic powder has progressed in recent years. However, when the quantity of polar groups introduced into the binder to increase the dispersibility of the microparticulate powder is increased, association of polar groups may conversely diminish dispersibility. Accordingly, the simple introduction of polar groups tends not to ensure adequate dispersibility.
(2) A good running property permitting repeated use is an example of a characteristic that is required of magnetic recording media. However, conventional vinyl polymers tend not to form high-strength films by themselves. Coating strength has been ensured by employing them in combination with polyurethane resin (see Documents 2 to 4, for example).
As set forth above, despite advantages such as a high degree of unit freedom and ease of conducting the synthesis reaction, vinyl polymers have tended not to achieve both good electromagnetic characteristics and running durability in conventional high-density recording-use magnetic recording media.